Converter systems are used in a wide variety of applications. A converter system generally includes a converter unit. The DC side of the converter unit is connected to a DC circuit. The AC side of the converter unit is generally arranged for connection to an AC electricity system and/or an electrical load, such as a rotating electric machine or a transformer, whereby the connection of any electrical load is conceivable. Depending upon the application concerned, the connection of a shorting circuit on the AC side is also customary. A converter system of the above-mentioned type, with a shorting circuit, is disclosed, for example, in US 2005/0281065 A1 and in US 2007/0291426 A1. If the current-carrying capacity of the converter unit is exceeded, the switches in the shorting circuit will close, and the AC side of the converter unit is short-circuited by means of a shorting resistor in the shorting circuit.
Given the simultaneous combination of requirements for a high current-carrying capacity with low component costs, the use of thyristors as switches in the shorting circuit, as disclosed, for example, in US 2007/0291426 A1, is a rational option. However, thyristors cannot be actively tripped or turned off. The turn-off of a thyristor is only possible upon the zero crossing of the current to be carried, thereby necessitating the use of additional and expensive “turn-off circuits”.
In addition to their application in rotating frequency converters, electrical loads in the form of double-fed asynchronous machines are increasingly used in variable-speed pumped storage power plants. In many locations, the “unbundling” of electricity generation and transmission functions is, or has been associated with the introduction of a Grid Code, in which the relevant conditions for connection to the electricity supply system are/have been defined. In some cases, new and more stringent conditions are to be fulfilled in response to system disturbances, specifically voltage dips. During transient voltage dips, it is intended that connected power plants and frequency-conversion installations should remain connected to the electricity supply system and, in some cases, will deliver an active voltage-stabilization function. This is achieved by the controlled in-feed of reactive current, in accordance with the provisions of the relevant Grid Code. In case of voltage dips, converter units for the supply of alternating current to the rotor windings of double-fed asynchronous machines will be temporarily exposed to a current which exceeds the normal service current. In order to protect the converter unit against overloading and, at the same time, to restrict the rotor voltage to an acceptable level for the rotor of the machine concerned, the above-mentioned shorting circuits are provided with a plurality of thyristors. Shorting circuits of this type provide reliable protection for the converter unit and its associated machine by short-circuiting the rotor winding in case of a fault. However, where shorting circuits with thyristors of this type are used, the immediate restoration of normal service—regulated by the converter unit—the current is reduced to an acceptable value which cannot be achieved within a short space of time.
Further generic instances of known techniques are disclosed in “Ride through of Wind Turbines with doubly fed Induction Generator under symmetrical Voltage Dips”, IEEE Transactions on Industrial Electronics, 2009, in WO 2004/091085 A1, in WO 03/065567 A1, in EP 0970840 A2 and in EP 1780856 A2.